Deglacial and Holocene sediment distribution in Hestvatn, South Iceland, derived from a seimic and multibeam survey

Abstract

More than 100 km of seismic reflection profiles together with a new multibeam survey of the sediment fill in lake Hestvatn, South Iceland, reveal two sub-basins filled with up to 44 m of deglacial and Holocene sediment. The chronology is constrained by geochemically characterized tephra layers and radiocarbon dates on marine molluscs. The Vedde Ash and Saksunarvatn Ash provide key chronological control for the deglacial sediment. Five seismic units are tied to the lithostratigraphy of sediment cores obtained from the two basins. The seismic units reveal major changes from glacial, to glacial-marine, to lacustrine sedimentary environments. Isopach maps of the seismic units document changes in the primary sediment depocenters, sedimentation rates and sediment delivery through time. The Vedde Ash, found only in glacial-marine sediment in the southern basin, suggests that during the Younger Dryas the northern basin of Hestvatn was occupied by an outlet glacier front the main Icelandic Ice Sheet that calved into a marine embayment. A high-resolution multibeam bathymetric survey reveals multiple shallow ridges between the two sub-basins of Hestvatn, which presumably acted as a pinning point for the calving glacier. During and after the retreat of the glacier and isolation of the lake basin, lacustrine sediments were mainly deposited via northern and northwesterly inlets, resulting in higher sedimentation rates in the north basin compared to the south basin. Early in the lacustrine phase repeated turbidites were deposited, interpreted to originate in glacial outburst floods (jokulhlaups) following a retreating ice-margin in central Iceland.